1. Field of Invention
This invention relates generally to a hermetic packaging technology for a two terminal silicon die such as a Schottky rectifier having a top and bottom solder contact area. More particularly, the invention relates to such a technology comprising a pressed ceramic frame, solid metal pads, a solid metal disc, metal seal rings, and a direct high temperature solder bond to the silicon die on both sides. Furthermore, a technology wherein the device is hermetically sealed and all materials are substantially the same coefficient A E D expansion (CTE).
2. Description of Related Art
Schottky devices are used extensively in power supply and converter outputs for military and space systems. These high reliability applications require the use of a hermetically sealed package to prevent moisture from contacting the die and degrading electrical performance. There is a great need for hermetic Schottky devices in small surface mount packages to reduce the size and weight of the circuit.
Most prior designs are based on creating a separate, stand-alone hermetic package which is assembled first, then the die is attached into the package. Prior to the present invention, all designs utilized a package having straps, wires, or clips to attach the silicon die to the metal pad serving as the final contact to the circuit. The strap design is limited to the current density and thermal conductivity of the strap. The installation of the device to a circuit board using the strap design must be in a particular direction, such that the die is at the side of the package attached to the circuit, the end opposite the strap. Otherwise, the strap would limit the thermal conductivity from the die to the circuit board. This also means that reverse polarity devices must be made with the die mounted upside down inside the package, therefore the same device can not be used for different polarities.
Numerous U.S. Patents exist for hermetic surface mount packages which can be used to accommodate silicon die. Saint-Cyr U.S. Pat. No. 4,879,589 discloses a hermetic leadless (surface mount) package for Schottky die. The Saint-Cyr invention is the only previously known art which does not attempt to create a separate, stand-alone hermetic package. The invention uses metal contact pads, a ceramic frame, and a J-shaped spring contact which is welded to one of the pads. The Schottky die is then soldered to the other end of the J-shaped spring. This subassembly is then soldered together at one time with the other metal pad and the ceramic frame. There are several limitations of this design. The most serious concern is that this is a "blind" construction. It is not possible to accurately verify that the die has made intimate contact to the other metal contact pad during assembly. It is conceivable that during assembly the die may not completely bond to the other contact pad. For a high current Schottky device this is not acceptable since it will directly and negatively affect thermal resistance. Another limitation is that the size of the J-spring limits the electrical conductivity of the device. This may not be a concern during normal bias if the current is substantially low, however the J-spring directly limits the surge capability of the device, which is an important attribute of a Schottky or any other power device. The J-spring construction allows the device to be mounted only in one direction on the circuit board. If the device is mounted with the J-spring side adjacent to the circuit, the thermal path will be limited to the J-spring only which will increase the thermal resistance by many times. If a reverse polarity device is required, this constitutes a different device wherein the other side of the die is mounted to the J-spring. This is not convenient or cost effective since devices must be made specifically for the application.